The invention relates to an evaporating crucible in which the material to be evaporated is fed in the form of rods through the crucible floor, and which has a rim set back from the circumference of the rods.
The bottom feeding of evaporating crucibles with material in rod form for evaporation is known through U.S. Pat. No. 3,607,222. The end surface of the rod is bombarded by an electron beam, so that a molten pool forms at the point of impingement of the electron beams, from which the evaporation takes place. The inside cross section of the known evaporating crucible, however, is substantially the same as the rod cross section. Such evaporating sources consequently have a point action, i.e., the cloud of vapor rising from the evaporation source has a limited horizontal cross section, so that is is not possible to dispose a plurality of substrates in the vapor stream much less produce thereon coatings of uniform thickness and of uniform composition throughout. A considerable irregularity persists even when a plurality of such evaporation sources are arrayed in what is termed an "evaporator field."
It is furthermore known to enlarge the cross section of the mouth of the crucible in the area of the molten pool at the upper end of the rod to a multiple of the rod cross section. In this case the molten pool extends horizontally beyond the circumference of the rods fed through the bottom. In this case, too, a multiple array of such evaporation sources cannot produce a vapor stream that is in any way uniform or in which a plurality of substrates can be disposed on which uniform coatings are to be produced. It must be considered that, as the diameter of the aperture of the crucible increases, the horizontal spacing of the plurality of crucibles would have to increase, thereby worsening the degree of irregularity.
There is another problem in water-cooled evaporating crucibles, in which a molten pool or bath is used, in that a solidified metal layer forms on the inside surface of the evaporating crucible, which is called a "dish" or also a "skull." This solidified metal layer has the undesirable effect that it prevents interaction between the material being evaporated and the evaporation crucible. When the material is fed in rod form through the bottom, however, the solidified layer produces the additional problem that it occasionally sticks to the rods, so that the dish-shaped solidified material is lifted together with the infed rods. A cavity is thus formed under the dish, into which a part of the molten metal flows as soon as the dish is locally melted through on account of the alteration of the heat balance through the removal of the dish from the cooling action provided at the walls of the crucible. At unforeseeable intervals, therefore, a considerable portion of the material being evaporated fails to participate in the evaporating action, resulting in irregularities in the coating process, since the rate of condensation is necessarily dependent on the rate of evaporation.